Gas defrost systems are common in commercial refrigeration devices to melt ice and frost accumulating on the evaporators. Generally, gas defrost systems redirect the superheated refrigerant discharge vapor from the compressors in a reversed direction through the evaporators. Recirculating the superheated refrigerant vapor defrosts the evaporators with the vapor desuperheating and condensing before exiting the evaporators. The refrigerant is then reintroduced into the main liquid distribution system at the liquid supply manifold or at the receiver tank of the refrigeration system.
The greater portion of the defrosting is accomplished through the latent process of condensation rather than a sensible heat exchange. In the latter stages of the .defrost cycle, the temperature differential between the refrigerant and evaporator decreases; therefore, not all of the refrigerant undergoes a complete phase change. Consequently, the refrigerant reentering the liquid distribution system has a mixed-phase condition, part liquid and part vapor. Since the main liquid distribution system is still feeding liquid refrigerant to other branch circuits (multiple evaporators) in the cooling mode, the reintroduction of a two phase refrigerant to the main liquid distribution system during the defrosting operation results in temporary inefficiencies. The dual phase refrigerants also causes the compressors to pump longer and harder during a defrost cycle and can also temporarily elevate cabinet temperatures.